1. Field of the Invention
The present invention relates to an automatic decompression system which functions to reduce the starting torque of the engine for starting it, and which is improved in the sensitivity, powerfullness of the decompression performance, in the smallness, and in the durability.
The present invention especially relates to such automatic decompression system which is provided with a common construction shown in FIGS. 1 to 8 of the attached drawings.
Namely, in the system, a cam shaft 2 of valve moving system 1 of the engine comprises a cam 3 and a flange 4 disposed thereon and properly separated from each other. And a cam gear 5 is fitted outwardly to the cam shaft 2 and contacted with the end surface 6 of the flange 4 at the contrary side to the cam 3. A guide hole 8 is provided to penetrate the shaft part 7 transversely between the cam 3 and the flange 4. A decompressing pin 10 is provided to pass reciprocatable through the guide hole 8. A fly weight 12 is pivoted on the cam gear 5 at the cam side surface 11, receiving the input part of the decompression pin 10 at the output part 15 thereof. The fly weight 12 is biased by a spring 14 to push the decompression spring 10 on the decompression position A at where the tappet 18 is pushed up by the tension of the spring 14, and the fly weight 12 is forced by the centrifugal force of itself to the cancelling position B at where the pin 10 is withdrawn from the tappet 18.
2. Related Art
As the first example, such system is found in the Japanese Patent Issue No. 46-39,892. As seen in FIGS. 7 and 8, the system described therein has a fly weight 12 which is located in an annular groove 52 formed between the rim 20 and the boss 51 of the cam gear. The outward motion of the fly weight 12 is stopped by the inner periphery of the rim 20, and the inward motion by the outer periphery of the boss 51. The output part 15 of the fly weight 5 presents a slit form and the input end part 16 of the decompression pin 10 is folded three times in right angle every time to fit in the output groove 15 of the fly weight 12. The klanking part 53 of the end part 16 near the output part 15 is inserted in a guide groove 54 formed radially in the flange 4.
In this known structure, the fly weight 12 is arranged within the annular groove 52 of the cam gear 5, so that it is advantageous to arrange smaller in the length along the cam shaft 2 by minimizing the space between the cam 3 and the cam gear 5.
However, the width 55 of the annuler groove 52 and the amplitude of the fly weight 12 is limited so small that the lifting angle (tangential inclination angle of the flank) necessary for lifting up the decompression pin 10 to carry out the decompression function must become stern and that the frictional resistance between the fly weight 12 and the pin 10 must become heavy. Moreover, the finishing accuracy can not be sufficiently high, since the output groove 15 is formed by internal machining, and the frictional resistance would become heavier by the roughness of the finishing. Furthermore, the decompression pin 10 receives heavy frictional resistance by the hard contact with the surface of the guide groove 54 formed in the flange 4, for the pin 10 is tending to turn about the guide hole 8 when it is driven by the fly weight 12.
Thus, the transmission efficiency between the fly weight 12 and the decompression pin 10 is low, the power to drive the decompression pin 10 is weak, and the sensitivity for cancelling the decompression function is insensible for the heavy frictional resistance of three kind abovementioned. Moreover, the fly weight 12 and the decompression pin 10 are easily defaced at their contacting faces by the heavy friction between them.
The second example is found in the Japanese Utility Model Issue No. 51-41,973. This example is comprised as described below and as shown in FIGS. 5 and 6.
Namely, the fly weight 12 is located outside of the cam gear 5, and the output part 15 is formed in extruding arc with a small curvature radius. The territory of swinging motion of the fly weight 12 is limited by a gap defined by the stopper pin 61 within a slit 62. The stopper pin 61 is fixed to the cam gear 5, and the slit 62 is cut through the fly weight 12.
In this structure, more accurate finishing accuracy is available and the frictional resistances may be decreased for that the finishing of the output part 15 of the fly weight 12, which presents an extruding curvilinear surface, may be machined by external machining. And it is advantageous in decreasing the heavy friction of decompression pin 10 with the guide groove 54 of the previous example.
However, in this system the curvature radius of the output part 15 of the fly weight 12 is so small that the swing of the fly weight is small dimentionally, and that the lifting angle is stern, therefore, the output part 15 still recieves heavy frictional resistance. Consequently, the problems of the weakness of decompression power, insensitivity in cancelling of the decompression, and the easiness of the defacement at the output part 15 of the fly weight 12 are still remaining.
Moreover, the length along the cam shaft becomes large by the largeness of the space between the cam gear 5 and the cam 3, since the fly weight 12 is located out side of the side surface of the cam gear 5.
Furthermore, the fly weight is easily damaged at the portion where the slit 62 is cut through, decreased the strength by the slit 62.